The effects of soil compaction

Soil compaction is an emerging challenge facing the livestock industry. A survey of soil quality in New Zealand found low macroporosity arising from moderate compaction in many pasture soils, with half the dairy sites, predominantly in the more resilient soils (having a macroporosity of <10%).

The loss of macropores impacts on pasture growth through reduced soil aeration and drainage and increased gaseous losses of N. Spring pasture growth can improve by 1.5% per unit of macroporosity between a macroporosity of 5–15%.

We therefore initiated a project to examine the impact of soil compaction on pasture in response to added phosphorus fertiliser, using sites in the Manawatu and Southland. Results suggest that the negative impact of a compacted soil on pasture growth can be offset to some degree by lifting the soil P status (the Olsen P value).

Our work suggests that the inclusion of a measure of soil physical condition in either the derivation of the Olsen P relative yield curves or in the interpretation of the Olsen P test value warrants further study. This is important, given the increasing cost of this nutrient, and the potential impact that soil with limited pore function and elevated Olsen P could have on surface water quality.